Petra Diamonds Ltd lowered its production forecast for fiscal 2013, due to a revision of the mining scope at its Finsch mine in South Africa and lower production at its Williamson mine in Tanzania.

The company, which has interests in seven producing mines in South Africa and one in Tanzania, said it expects to produce about 2.9 million carats for the year ending June 30, 2013, a quarter of a million carats lower than its prior view.

It produced about 2.2 million carats in fiscal 2012. Petra said its production target of 5 million carats a year by fiscal 2019 remained on track. The company said it expected diamond prices to remain flat for the rest of this year.

"The rough diamond market is expected to remain under pressure in the short term, however, Petra's view is that the medium to long-term outlook for diamond prices remains positive due to the well-reported medium-term strong supply and demand fundamentals," the FTSE 250 company said in a statement.

The Finsch mine, South Africa's second-largest diamond mine by production which Petra bought from De Beers in September, contributed 1.1 million carats to overall output in the year ended June 30. Petra shares closed at 99.5 pence on Tuesday on the London Stock Exchange.